Transistors such as metal oxide semiconductor field effect transistors (MOSFETs) or simply field effect transistors (FETs) or MOS transistors are the core building blocks of the vast majority of semiconductor integrated circuits (ICs). A FET includes source and drain regions between which a current can flow through a channel under the influence of a bias applied to a gate electrode that overlies the channel and that is separated from the channel by a gate dielectric structure. The ICs are usually formed using both P-channel FETs (PMOS transistors or PFETs) and N-channel FETs (NMOS transistors or NFETs) and the IC is then referred to as a complementary MOS or CMOS circuit. Some semiconductor ICs, such as high performance microprocessors, can include millions of FETs. For such ICs, decreasing transistor size and thus increasing transistor density has traditionally been a high priority in the semiconductor manufacturing industry. Transistor performance, however, must be maintained even as the transistor size decreases.
As advanced metal-oxide-semiconductor (MOS) technology continues to scale and move into the deep-sub-micron geometry dimensions, advancements to techniques for further decreasing critical dimension (CD) of features have been sought. Self-aligned double patterning (SADP) is one technique that is used to pattern sub-resolution features in integrated circuits and often results in less line width roughness and enhanced control of CDs as compared to conventional lithography/etching techniques. During SADP, spacers are generally formed over an underlying substrate by first patterning mandrels over the underlying substrate. Spacer material is deposited over the mandrels and the underlying substrate and the spacer material is etched from horizontal surfaces to leave spacers adjacent to sidewalls of the mandrels. The mandrels are then removed, followed by etching through gaps between the spacers into the underlying substrate to thereby transfer a pattern into the underlying substrate. Features of the spacers define the pattern that is ultimately transferred into the underlying substrate, and sub-resolution pitch of the spacers can be achieved that is not possible through patterning of the mandrels with conventional lithography/etching techniques. The patterned underlying substrate may be employed for further patterning, or may be included in a final integrated circuit. For example, SADP may be employed for gate-level patterning, fin level patterning in FinFETs, or other patterning stages during both front-end-of-line (FEOL) and back-end-of line (BEOL) processing.
Conventional SADP generally only provides a nominal CD due to the uniform formation of the spacers, with all spacers formed through SADP generally having the same thickness. To provide spacers having different thicknesses, which may be desired to fabricate multi-gate length configurations, i.e., configurations that include nominal and biased gate lengths, in integrated circuits, additional masks can be selectively used to isolate spacers that are to be modified for purposes of defining different gate lengths, also known as gate length biasing. Gate length biasing enables current leakage between gates to be minimized, thereby minimizing power consumption of the integrated circuit although device performance/speed is sacrificed. In accordance with such techniques, a separate mask is required for each additional gate length. Modification of the spacers can be conducted through additive techniques, by which additional material is formed over the spacers to increase a width thereof, or by subtractive techniques, by which spacer material is removed (such as through etching) to decrease the width thereof. The additive and subtractive techniques may be conducted either before or after the mandrels are removed. However, additive and subtractive techniques for spacer modification present various problems. For example, subtractive techniques cannot be reliably implemented on a commercial scale because blind etching of the spacers is generally unpredictable, especially when the spacers are to be thinned by less than about 5 nm. Further, CD uniformity (CDU) is a challenge using the subtractive techniques. Regarding additive techniques and depending on different approaches, either variable mandrel CD and pitch is required (which is a challenge for both OPC and mask manufacturing) or multiple additional processing stages are required, thereby increasing processing cost.
Accordingly, it is desirable to provide methods of forming integrated circuits and multiple CD SADP processes that can be reliability implemented on a commercial scale with maximized CDU, especially when modification of the spacer CDs is on a small scale. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.